Having been in a similar position more times than I care to count, all I can say with universality is that there is no correct answer.

Part of it depends on what her feelings are about abortion. Some talking heads would like us to believe it's strictly a medical decision for the woman; if she really believes that, then there really isn't anything that's worth saying; "I'm sorry" carries the kind of moral implication that she might find offensive, making it ill-advised unless you're both chomping at the bit to have a debate. However, it has been my experience that most women understand instinctively that there's more to it than that, and as such depression is a common reaction. She may be looking for support and comfort, someone to tell her that she is not the most heinous anti-mother in the history of the species. "The worst day of my life" has been a common phrase in my experience. In those cases, and I believe (assertion without proof) this to be the significant majority of cases in which the woman would actually tell someone with whom they're not on intimate terms, "I'm sorry" can be a good starting point - the ambiguity of the phrase in american parlance is helpful, in that it expresses (in a non-specific way) a sympathy for the difficulty (and even traumatic nature) of the experience, regret that a situation even arose for which this was a potential recourse, and so on.

Of course, I say "starting point" assuming this conversation should go somewhere - which may not be the case. How much is appropriate to be said will depend upon the depth of your friendship. The wisdom of getting into what could easily become a kind of grief counseling session with an acquaintance from calc I is questionable at best. The whole college dorm culture can be rather dangerous in this way, in that there is often something of a shortage of friends with long track records with whom it would be relatively safe to share such things, and lacking such an appropriate venue, people will sometimes reach out to anyone who will listen. This is both dangerous for them and you, so tread cautiously, and take advantage of the university's counseling center early and often.

BTW, here's a tip for all of you argumentative noders out there: odds are, if she knows you well enough to be sharing this with you, she also knows (or at least has an idea) of where you stand on the legality, morality, and ethics of abortion, so turning to a discussion of that will not generally be very productive. Few things will harden someone against you quite like an assault with logic when they're emotionally vulnerable. On the flip side of that coin, when engaging in public debate about aforementioned legality, morality, and ethics, nothing brings reasonable discourse to a screeching halt quite like standing up and shouting "How dare you judge me?" - think of it as a kind of Godwin's Law for public debate on the matter.

"Sometimes the wisest thing to say, is nothing"

In my case, there was little I could say. My ex-partner made the reluctant decision to terminate her pregnancy following our break-up. Despite our desire to keep and support our child, she bowed to enormous pressure from both our families (including my Christian parents). She phoned me to tell me of her resolve, and my own feelings and desires were ignored and crushed. Nonetheless, despite my personal feelings, I still supported her.

Knowing what to say was almost impossible. For her, she felt she was in a no-win situation (as I imagine many women are), and as she was studying midwifery, one which cut to the very heart of her identity. She was also already a mother of four, and had lost one to Sudden Infant Death Syndrome.

For me, it was heart-breaking. I had always wanted to be a father, had loved my brief time as the man in her surviving children's lives, and still loved her. But the relationship was over, and she had made her decision regarding the abortion.

All I could do was nod, and weep.

Yes, you may think it's off-topic, but it's my way of saying that sometimes you can say nothing...

anon says thank you for sharing what do you say to someone . . . Having been the female in such a situation, I can say that it's difficult on the woman to make a unilateral decision, but you often feel you have no choice
liveforever says I just read your WU What do you say to someone who has just had an abortion? and wanted to express my sympathy. As a father myself, I can understand how you must feel.
anon says my heart goes out to you - I had an abortion a long time ago, so I really do know how much anguish and pain it involves. I'm so sorry.
anon says no-one ever wants an abortin (sic), and to talk about it is so difficult. For some it's a relief have one, I'm so sorry it wasn't for you
anon says Me and my girl tried to conceive a baby. We succeeded, and then my father basically said he would disown me if it was carried to term. Sad, sad, sad.